Round duct: CFM from diameter and velocity
Inputs: 8 in round at 600 FPM.
Calculation: Area π(8/12)²/4 multiplied by 600 FPM.
Result: 209.4 CFM.
Airflow and velocity
Duct Airflow and Velocity Checker
Enter duct dimensions and CFM or air speed to solve for airflow, velocity, or capacity.
Concepts behind this calculation
Check airflow, average velocity, a selected velocity limit, rectangular area change or aspect ratio from the existing duct dimensions.
Airflow is the volume crossing a duct section each minute. Multiplying clear area by an area-average velocity gives volume flow; a centerline reading can overstate that average when the velocity profile is uneven.
A velocity-limit check compares geometry with a selected design criterion. Changing cross-sectional area at unchanged CFM changes velocity, while an aspect-ratio check describes shape rather than pressure performance.
How to use this calculator
- Select round or rectangular CFM when average velocity is known; select the matching velocity mode when CFM is known.
- Enter clear inside diameter or inside width and height. For the velocity-limit check add the selected velocity limit; for area change enter both upstream and downstream dimensions at the same airflow.
- Use the aspect-ratio comparison only for the selected maximum width-to-height ratio. Keep this geometry result separate from a pressure-loss or fan-capacity decision.
Calculation method and applicability
Q = V·A; V = Q/A; round A = π(D/12)²/4; rectangular A = WH/144
Q = volume flow (CFM); V = average velocity (FPM); A = clear area (ft²). D, W and H are inside dimensions in inches, converted by the displayed 12 or 144 divisor.
Input definitions and units
- Calculation
- Choose the relationship that matches the known airflow, geometry and decision. Choices: Round duct: CFM from diameter and velocity; Rectangular duct: CFM from width, height and velocity; Round duct: velocity from CFM and diameter; Rectangular duct: velocity from CFM, width and height; Round duct: compare velocity with a selected limit; Rectangular duct: compare two section areas and velocities; Rectangular duct: compare aspect ratio with a selected limit.
- Inside diameter (in)
- Measure the clear inside diameter, including any liner effect. Applies to Calculation: Round duct: CFM from diameter and velocity.
- Average or selected velocity (FPM)
- Use an area-average measured velocity or an explicitly selected design target. Applies to Calculation: Round duct: CFM from diameter and velocity.
- Inside width (in)
- Clear inside rectangular width. Applies to Calculation: Rectangular duct: CFM from width, height and velocity.
- Inside height (in)
- Clear inside rectangular height. Applies to Calculation: Rectangular duct: CFM from width, height and velocity.
- Average velocity (FPM)
- Use an area-average measured velocity or an explicitly selected design target. Applies to Calculation: Rectangular duct: CFM from width, height and velocity.
- Airflow (CFM)
- Use airflow at the stated measurement/design point. Applies to Calculation: Round duct: velocity from CFM and diameter.
- Inside diameter (in)
- Measure the clear inside diameter. Applies to Calculation: Round duct: velocity from CFM and diameter.
- Airflow (CFM)
- Use airflow at the stated point. Applies to Calculation: Rectangular duct: velocity from CFM, width and height.
- Inside width (in)
- Clear inside rectangular width. Applies to Calculation: Rectangular duct: velocity from CFM, width and height.
- Inside height (in)
- Clear inside rectangular height. Applies to Calculation: Rectangular duct: velocity from CFM, width and height.
- Airflow (CFM)
- The limit comparison uses the geometry-derived average velocity. Applies to Calculation: Round duct: compare velocity with a selected limit.
- Inside diameter (in)
- Round duct clear inside diameter. Applies to Calculation: Round duct: compare velocity with a selected limit.
- Selected velocity limit (FPM)
- A user-selected engineering criterion, not a universal code limit. Applies to Calculation: Round duct: compare velocity with a selected limit.
- Airflow (CFM)
- Assumed unchanged across the compared sections. Applies to Calculation: Rectangular duct: compare two section areas and velocities.
- Upstream inside width (in)
- Clear inside width before the transition. Applies to Calculation: Rectangular duct: compare two section areas and velocities.
- Upstream inside height (in)
- Clear inside height before the transition. Applies to Calculation: Rectangular duct: compare two section areas and velocities.
- Downstream inside width (in)
- Clear inside width after the transition. Applies to Calculation: Rectangular duct: compare two section areas and velocities.
- Downstream inside height (in)
- Clear inside height after the transition. Applies to Calculation: Rectangular duct: compare two section areas and velocities.
- Inside width (in)
- Clear inside rectangular width. Applies to Calculation: Rectangular duct: compare aspect ratio with a selected limit.
- Inside height (in)
- Clear inside rectangular height. Applies to Calculation: Rectangular duct: compare aspect ratio with a selected limit.
- Selected maximum aspect ratio (:1)
- Use a project/material constraint; no universal pass threshold is applied. Applies to Calculation: Rectangular duct: compare aspect ratio with a selected limit.
How to interpret the result
Read CFM as operating airflow only when velocity is measured and representative. A selected velocity cap produces theoretical geometric capacity; aspect and velocity flags apply only to the entered limits.
Worked examples
Rectangular duct: CFM from width, height and velocity
Inputs: 12 × 8 in at 600 FPM.
Calculation: Area 12×8/144 ft² multiplied by 600 FPM.
Result: 400 CFM.
Round duct: velocity from CFM and diameter
Inputs: 400 CFM through a 10 in round duct.
Calculation: Divide flow by the clear round area.
Result: 733.4 FPM average velocity.
Rectangular duct: velocity from CFM, width and height
Inputs: 400 CFM through a 12 × 8 in rectangular duct.
Calculation: 400/(12×8/144).
Result: 600 FPM average velocity.
Round duct: compare velocity with a selected limit
Inputs: 400 CFM, 10 in round duct, selected 700 FPM limit.
Calculation: Geometry gives about 733.4 FPM.
Result: Above the selected limit; this is not a code verdict.
Rectangular duct: compare two section areas and velocities
Inputs: 400 CFM through 12×8 in then 8×8 in.
Calculation: Compute clear area and average velocity at both sections.
Result: 600 FPM → 900 FPM; area ratio 0.667.
Rectangular duct: compare aspect ratio with a selected limit
Inputs: 24 × 6 in rectangle; selected limit 4:1.
Calculation: Width divided by height is 4:1.
Result: At the selected limit.
Assumptions and limits
- Average velocity must represent the cross-section; a single centerline reading is not an area average.
- A selected velocity limit is an input criterion, not a universal code maximum.
- Geometry and velocity checks do not calculate fitting loss, fan pressure, noise, leakage or equipment operating limits.
FAQs
Does a larger duct create more airflow?
At the same imposed airflow it lowers average velocity. Actual operating airflow also depends on fan performance and system resistance, which this section check does not solve.
Should I measure the outside of lined duct?
Use the clear inside opening after liner and obstructions. Outside sheet dimensions overstate the air passage and therefore overstate CFM calculated from velocity.
Sources
- ASHRAE 2025 Fundamentals, Chapter 21: Duct DesignDuct design context. The listed geometry relationships are direct dimensional calculations.
Related calculators
Related reading
Measured airflow balancing: compare, adjust, and remeasure Use a complete simultaneous terminal schedule and explicitly selected criteria to separate design-band checks, group totals, and proportional balancing references. CFM, FPM, duct area, and airflow capacity Use clear inside area and representative average velocity to solve airflow, or use airflow to find average velocity for one duct section. Duct design workflow: from airflow to a pressure check Connect an established airflow requirement to clear duct geometry, constructible routes, compatible losses, and an equipment pressure boundary in a traceable sequence. Choose the duct calculation that matches the job Start with the decision you need to make, then choose the airflow, sizing, pressure, component, or system check that supports it. Plan and interpret a duct traverse Plan measurement points from clear duct dimensions, collect representative readings, and reduce them using the matching instrument method.